It may be possible to treat congenital enzyme deficiencies or hepatocellular degenerative conditions by transplanting liver cells isolated from another individual into the liver of affected individuals. To investigate such a possibility, it is proposed that hepatocytes will be isolated from heterozygous Gunn rats and Wistar rats; they will be radioactively labeled in vivo or in vitro and transplanted via the portal vein into the liver of homozygous, jaundiced Gunn rats. The transplanted cells will be monitored with light and electron microscope autoradiography for three months or more to obtain histological evidence of the fate of the transplantated cells and the host responses. Pre-and post-operative plasma bilirubin concentrations in the recipients will be measured to obtain functional evidence of the ability of the transplanted hepatocytes to correct the congenital deficiency of glucuronyl transferase in the jaundiced transplant recipients. The feasibility of performing syngeneic, allogeneic or xenogeneic transplantation of hepathocytes will be compared by transplanting labeled hepatocytes from Gunn rats, Wistar rats, mice and temporarily maintained cultures of hepatocytes from various species into jaundiced Gunn rat recipients. Isolated, labeled, rabbit hepatocytes will be transplanted via mesenteric or umbilical veins into recipient new born and developing rabbits to investigate the potentialities of these routes as alternatives to direct intraportal transplantation in larger animals and man.